A microwave intrusion detecting device, which is a so-called electronic fence, is used to detect whether an area is intruded by an object. Detection utilizes emitting electromagnetic waves and compares energy of transmitted electromagnetic waves with energy of received electromagnetic waves to determine whether an intrusion occurs. There are two main types of detection method. One detection method is an on-beam type electronic fence, which is a so-called bistatic electronic fence as shown in FIG. 1, where a transmission end 102 and a receiving end 104 of the electronic fence are installed in different locations. The detection area 120 is between the transmitting device and the receiving device. The energy detected by the receiving end from the transmitting end will decrease when objects intrude into the detection area 120 thus intrusion is determined. Another detection method is a radar-type electronic fence which is a so-called monostatic electronic fence as shown in FIG. 2. The transmitting end and the receiving end of this type of electronic fence are located in the same device 200. The measurement area 220 is determined by the angle of the antenna and the energy intensity of the emitted electromagnetic wave. When an object intrudes into detection area 220, the electromagnetic wave is reflected back to the detecting device 200, so that the electromagnetic wave energy detected by the receiving end will rise thus intrusion can be determined.
The bistatic electronic fences require being equipped with pair of transmitting devices and receiving devices at two different locations, thus installation cost is higher and the maintenance requires more effort. Additionally, the detection area is limited only between the transmitting device and the receiving device, and the transmitting device and the receiving device need to be aligned. In contrast, the monostatic electronic fences require being equipped with only one detection device. However, the detection area of the monostatic electronic fence is short. These two kinds of electronic fences are easily affected by environment or interfered by unintended signal reflections. In addition, in some cases, objects pass through certain parts of the electronic fence often, such as parking entrances or personnel sidewalk, thus false alarm occurs. The present solution is equipping passing objects or personnel with radio frequency identification chip. However, additional identification devices and identification chips are needed which results increase of the cost.
Therefore, a detecting device that combines a transmitting end and a receiving end is required in the purpose of reducing the installation cost. In the meanwhile, the detecting device itself can overcome environmental interference to reduce the probability of false detection.